Fix write_json to handle empty string column

[karthikeyann]

Description

Add empty string column condition for write_json
bypass make_strings_children for empty column because when grid size is zero, it throws cuda error.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[mhaseeb123]

Looks good. Should we add tests for this case as well as empty columns for other types?

[karthikeyann]

unit test added.

[karthikeyann]

Should we add tests for this case as well as empty columns for other types?

Not required. All other types use string functions which checks for empty inputs. Only custom function created in this file get_escaped_strings did not handle empty string column input. I added string, int empty column also to this unit test.

[davidwendt]

Nit on braces